Four Pillars 四柱
Sì Zhù · Year, month, day and hour, each written as one stem over one branch.
What it means
The Four Pillars are the four columns of a BaZi chart. Each pillar is a pair — one Heavenly Stem written above one Earthly Branch — and each covers a different cycle: the year pillar changes at Lichun, the month pillar at each of the twelve major solar terms, the day pillar every day at midnight in true solar time, and the hour pillar every two hours.
Why it matters
The four cycles run at wildly different speeds, and that is the point. The year pillar is shared by everyone born in the same solar year, the month pillar by everyone born in the same fortnight or so, the day pillar by roughly one person in sixty, and the hour pillar narrows it again by twelve. Reading them together is what makes a chart specific rather than generic, and it is why an error in the birth time can change a reading more than an error in the year.
Where this site uses it
The chart tool on this site prints all four pillars with the boundary that decided each one, so you can see which solar term set your month pillar and whether your birth time fell near a day change.